1. Field of Application
The present invention relates to a knock sensor signal processing apparatus which performs A/D conversion and digital filter processing of respective sensor signals produced from one or more knock sensors of an internal combustion engine.
2. Description of Related Art
Types of apparatus are known for use in controlling the engine of a vehicle whereby during respective intervals, a judgement is made as to whether knocking is occurring in a cylinder of the engine. The judgement is based upon an analog output signal (referred to in the following as a knock sensor signal) produced from a knock sensor that is installed in the engine, with the knock sensor signal being subjected to A/D conversion at fixed periodic intervals. Digital filter processing is applied the resultant train of digital values and judgement as to whether knocking is occurring in a cylinder is made based upon the results of the digital filter processing of these digital values.
In the following description and appended claims, a digital value resulting from an A/D conversion operation is referred to simply as an “A/D value”.
Such a form of knock judgement is described for example in Japanese patent first publication No. 2004-309267 (referred to in the following as reference document 1).
A signal processing apparatus for performing such A/D conversion and digital filter processing of a knock sensor signal can for example be based on a first apparatus that includes an A/D converter which operates on the knock sensor signal and a second apparatus which acquires resultant A/D values from the first apparatus at periodic fixed intervals, transmitted via a communication line, with the second apparatus performing digital filter processing of the series of sensor A/D values and knock judgement based on the filtering results. With such a system, the second apparatus may transmit commands to the first apparatus that include request commands for designating respective A/D conversions to be performed (with these commands being sent at fixed periodic intervals). Each time that the first apparatus receives such an A/D conversion request command, it transmits an A/D value obtained from the A/D converter, to the second apparatus. A/D values are thereby transmitted from the first apparatus to the second apparatus at regular intervals. Such a system has been described by the assignees of the present invention, in Japanese patent first publication No. 2006-112346.
In the case of an engine having a plurality of cylinders, it is known to provide respectively separate knock sensors for each of a plurality of sets of cylinders. When a single system is used for performing knock sensor signal A/D conversion and digital filter processing with such a plurality of knock sensors, switching can be performed between inputting the output signals from the respective knock sensors to the A/D conversion and digital filter processing system. Such a system is described for example in Japanese patent first publication No. 9-60568 (referred to in the following as reference document 2).
An A/D converter for use with such a type of knock sensor signal processing apparatus should preferably have high speed of operation combined with high resolution of A/D conversion. An example of such an A/D converter is a pulse phase difference encoder circuit, which utilizes the relationship between the supply voltage applied to each of a plurality of delay stages formed of semiconductor devices and the amount of signal transmission delay of each delay stage. With such an A/D converter, the value of an analog input voltage applied as the supply voltage can be obtained as a count of the total number of delays stages traversed by a pulse signal during a fixed interval. Such a type of A/D converter is described for example in Japanese patent first publication No. 5-259907 (referred to in the following as reference document 3) and Japanese patent first publication No. 2005-223818 (referred to in the following as reference document 4), so that detailed description is omitted.
With such a type of A/D converter, high speed of conversion and high conversion resolution can both be achieved. However the A/D conversion characteristics vary in accordance with the operating temperature of the A/D converter, due to the relationship between the delay produced by each delay stage and temperature.
Furthermore, as shown in FIG. 1 of reference document 4, the input analog voltage of such a type of A/D converter may be supplied from an amplifier, with the central voltage level of the input analog voltage being thereby set as a reference voltage VR. However in that case, if variations occur in the level of the reference voltage VR, then resultant variations will occur in the A/D conversion characteristics, causing the accuracy of the obtained A/D values to be lowered, so that accurate judgement of occurrence of knocking cannot be achieved.